Fix #4145: Unable to view deleted comments from post page.

Bug: if all the comments on a post were deleted then the deleted
comments wouldn't be visible to moderators.

This was because we assumed that if `last_commented_at` was nil it meant
that the post had no comments, but this was wrong. `last_commented_at`
only counts undeleted comments. It's reset to nil if all the commnets
have been deleted.

app/controllers/posts_controller.rb

@@ -23,7 +23,7 @@ class PostsController < ApplicationController
   def show
     @post = Post.find(params[:id])
 
-    @comments = @post.last_commented_at.present? ? @post.comments : Comment.none
+    @comments = @post.comments
     @comments = @comments.includes(:creator)
     @comments = @comments.includes(:votes) if CurrentUser.is_member?
     @comments = @comments.select { |c| c.visible_by?(CurrentUser.user) }